NEW YORK (Reuters Health) - A scan of the Canadian population reveals that, thanks largely to fortified foods, almost no one is deficient in folic acid, and forty percent have relatively high -- maybe even too high -- levels of the B vitamin in their blood.

Despite the lack of deficiency, however, 1 in 5 women of childbearing age did not carry in their blood the amount of folate (the natural form of folic acid) recommended to prevent birth defects, the benefit for which the vitamin is best known.

So while many men, the elderly, and children may have a lot of folic acid, Canadian women of childbearing age often don't have enough, and should continue to take supplements, study author Cynthia Colapinto of the Children's Hospital of Eastern Ontario Research Institute told Reuters Health.

"I believe all women of childbearing age should take supplements," she said.

These findings are "reassuring," said Dr. Nicholas Wald, director of the Wolfson Institute of Preventive Medicine, who did not participate in the study. He cautioned that people should not focus on the high levels of folate found in the study, since there is still no evidence that these levels can cause problems.

"It is a mistake to talk about dangerously high levels of folate," Wald said in an e-mail to Reuters Health. "The important message is that folic acid fortification has worked and has been shown to be successful in improving health without evidence of harm."

Folate is the natural form of the B vitamin, found in foods such as spinach, asparagus, dried beans and peas, and orange juice; folic acid is the synthetic form used in vitamin supplements and added to certain "fortified" foods, including wheat flour and breakfast cereals.

Experts say that women need extra amounts of the vitamin to reduce the risk of having a baby with neural tube defects -- birth defects of the brain or spine, including spina bifida.

As a result, in the late 1990s, the U.S. and Canada mandated that folic acid be added to products such as flour to help prevent neural tube defects. It's also added to enriched cereal, pasta, corn meal and other grains. Since this change, the rate of neural tube defects in Canada has dropped by 46 percent.

To more closely investigate the effects of fortification, Colapinto and her team directly measured folate concentrations in the blood of 5,248 Canadians between the ages of 6 and 79, who were representative of the population as a whole.

The recommended daily intake of folate from all sources is 150 micrograms (mcg) for children ages 1 to 3; 200 mcg for ages 4 to 8; and 300 mcg for ages 9 to 13. Older teens and adult women and men are advised to get 400 mcg.

There is no established maximum recommended amount of folate. To estimate one here, Colapinto and her colleagues looked for people with circulating folate levels above those seen in the top 3 percent of Americans in a recent nationwide survey. That level, 600 nanograms per milliliter (ng/mL) of blood, is 50 percent higher than the 400 ng/mL experts consider optimal for women to prevent neural tube defects in their babies.

Reporting in the Canadian Medical Association Journal, they found that 40 percent of Canadians had blood levels of folate that exceeded 600 ng/mL (or 1360 nanomoles per liter, the measure more commonly used in Canada).

It's not clear exactly what the dangers of that much folic acid might bring, Colapinto said in an interview; some research suggests high levels of folic acid could worsen pre-existing cancers, or mask a deficiency of vitamin B12, which can lead to neurological problems.

The findings largely agree with those from a recent survey of more than 35,000 Canadians that estimated how much folate and folic acid individuals were consuming in foods, and how much additional folic acid they were getting from vitamin supplements. In that report, researchers also found that few Canadians are now getting too little of the B vitamin, raising the question of whether children and men need to get additional folic acid from vitamins.

But that previous study estimated a much lower rate of high folate levels nationally -- its authors suggested that 1 to 4 percent of Canadian children got too much folate when food and supplement sources were combined. The same was true for up to 5 percent of adults, depending on their sex and age group.

The prior study is "important research," Colapinto noted, but "slightly different" from her study, which looked directly at levels of folate in the blood, rather than estimating intake from diet and supplements. The disparate finding "certainly highlights the need to examine intake of folate-rich foods and folic acid-containing supplements in relation to blood folate concentrations, which we are currently investigating."

Even though 4 out of 10 Canadians in Colapinto's study had folate levels that exceeded those of 97 percent of Americans from a recent survey, that doesn't necessarily mean that Canadians are necessarily carrying around more folate than Americans, she cautioned. Each survey used different methods to measure folate levels, and the blood was collected during different time periods. "We have to be careful how we compare," she said.

Indeed, it is possible that similar trends may be occurring in other Western countries that have also begun fortifying foods with folic acid, Colapinto said. This paper "certainly is of interest to other countries," she said. "Whether or not it's generalizable to other countries, that needs to be explored further."

Researchers "could, and should" question the validity of the estimated maximum healthy intake of folate used in this study, Colapinto added. "We want researchers to challenge what we're doing."